Consider the sentence 桜の花が今年いつごろ咲くか発表がありました as being the conjunction of two sentences:
「か」is being used as a conjunctive particle in the particular case where the first sentence indicates a question.
Note: I have searched extensively for a reference listing か as a conjunctive particle. However, I could not find any such grammatical explanation. I could find one where it wasn't listed:
However, in all cases where か is serving as a conjunctive particle, it could be replaced by かについて、and について is a conjunctive particle. E.g.
- いついくか計画を作ろう → いついくかについて計画を作ろう
- 答えるかどうか決定せよ → 答えるかどうかについて決定せよ
I suppose you could say that after か the real conjunctive について is just being omitted, but even in that case then functionally speaking, か has come to serve as a conjunctive particle.
Post decision edit:
broken headphones answer gives the correct functional role, which is "adverbial clause", of 桜の花が今年いつごろ咲くか which modifies the predicate 発表がありました。I also recommend his explanation of "adverbial clause" vs. "adverb" found here.
A separate, but related, issue is the form used to "conjoin" the adverbial clause and the predicate, which also plays a role in expressing their relation.
For example, verb conjunctive form 「～て」、or conditional conjunctive form「～ば」.
**Do questions ending in ～か comprise a conjunctive form in their own right? **